The present invention relates to the field of producing unsaturated hydrocarbons, and more particularly to methods of producing butene-1.
Butene-1 can be used in the production of n-butyl alcohol, copolymers of ethylene with butylene, isotactic polybutene, butylene oligomers, as well as in the production of butadiene and other products of petrocnemical processing.
Several methods of producing butene-1 are known in the art: separation of butene-1 from the butane-butylene fraction of cracked gases, dehydration of butyl alcohol, dehydrogenation of butane, and thermal or catalytic dimerization of ethylene.
The process of producing butene-1 by dimerizing ethylene on complex organometallic catalysts has gained wide acceptance. In accordance with the known methods, dimerization of ethylene to butene-1 is carried out at temperatures ranging from 0.degree. to 100.degree. C, preferably from 10.degree. to 40.degree.C, with the pressure of ethylene being either atmospheric or elevated (up to 40 gauge atmospheres), in the medium of organic solvents (heptane, hexane, benzene, toluene, butane, isooctane, or mixtures of these solvents with butene-1).
In the processes of ethylene dimerization, complex catalysts including compounds of nickel, cobalt or titanium are employed.
Catalysts based on nickel or cobalt compounds are noted for their low selectivity.
Reactions of ethylene dimerization conducted with these catalysts give a complex mixture of products consisting of butene-1, cis-trans-butenes-2, hexenes, octenes, etc. The dimerization of ethylene on catalysts consisting of titanium alkoxides and organoaluminium compounds Ti(OR).sub.4 + AlR.sub.3 ' or AlR.sub.2 "H, where R, R', R" stand for an alkyl, cycloalkyl or aryl hydrocarbon radical in the medium of hydrocarbon solvents, usually results in the formation, along with butene-1, of from 0.5 to 5.0 vol. per cent of butenes-2 and from 1.5 to 8.0 weight per cent of polyethylene.
The dimerization of ethylene on catalysts of this type with an ethylene pressure of from 1 to 10 atm proceeds at a relatively low rate (1 to 2 g/lit per min. with the concentration of Ti(OR).sub.4 of about 5.sup.. 10.sup..sup.-3 mole/lit.). In the course of the process the catalyst rapidly loses its activity, which results in the lowering of its capacity, i.e. in a lower yield of butene-1 in moles per mole of Ti(OR).sub.4.
With a view to increasing the activity and capacity of the catalyst, the process of dimerizing ethylene to butene-1 is recommendably carried out at low temperatures (ranging from 10.degree. to 40.degree.C), this involving considerable difficulties as regards the problem of heat removal.
Low selectivity of the known processes of dimerizing ethylene to butene-1, low yields of the desired product per unit weight of the catalyst, sophisticated and cumbersome process equipment are all factors responsible for the high ultimate cost of butene-1. This is to a considerable extent associated with the fact that the formation of by-products, even in small amounts (2 to 5 weight per cent for the reacted ethylene) aoversely tells on the entire technological process of producing butene-1, since the by-products not only lower the yield of butene-1 and its purity, but also reduce the working time of the process equipment, insofar as solid polymer accumulating in the reactors has to be periodically removed, which can be done only by interrupting the dimerization process and, hence, at the expense of lost time of the equipment operation.
In some cases, for attaining a partial improvement of the process characteristics, it was suggested that the catalyst modifiers be introduced into the reaction mixture in amounts commensurable with the amount of the catalyst components (with the molar ratio modifier/Ti(OR).sub.4 being from 0.01 to 10; and modifier/AlR.sub.3 from 0.01 to 1.0).
As such modifiers organic esters of orthophosphoric acid, diphenylamine, phenothiazine, etc. may be used.
The inclusion of these compounds into the composition of the catalyst in amounts of from 0.1 to 1.0 mole per mole of the alkylaluminium taken leads to a certain reduction in the polymer formation; however, the activity and capacity of the catalyst are also essentially reduced in this case.
Different compounds used as modifying additives, even if they belong to the same class of compounds, produce different influence on both the rate of dimerization of ethylene to butene-1 and on the side reactions of the polymerization of ethylene to polyethylene. At present it is hardly possible to establish any definite relationship between the properties of the modifier (its structure, etc.) and the inhibitory effect it produces on the reaction of polymer formation, or its general influence on the process of dimerization of ethylene to butene-1. It is just for this reason that patents teach the application of modifiers of only individual compounds and not of whole classes of compounds.